SNI Support for the SMTP level was recently discussed in the Plesk Facebook Group. [1]
I suggest to implement Exim into Plesk as an alternative or addition to qmail/postfix.
- Logfiles are easier to parse/debug
- SNI Support at the SMTP helps to migrate end-customers from Server A to Server B without end-user having to change anything.
- Dovecot supports SNI, NGINX/Aapache Supports SNI but currently Postfix does not.
- Exim does.

Cpanel is the one 1 control panel right now and it uses exim. Try using cpanel and you will find more than 15 options and plugins to manage your email accounts, spam email management, virus/trojen detection etc etc.

The advantage for the Plesk developers is that Exim supports all Email features out of the box. They only have to be configured and/or activated. Exims powerful configuration with it's expansion language can support every thinkable requirement.
Postfix and Qmail on the other hand need custom handlers for many things.
If you should really be able to find something Exim can't do, you can still extend Exims configuration with Perl scripts.

The advantage for the server admin is the much simpler logfile format. Everything you need to know about a certain email is logged on one line for how it's coming in and one line for how it's going out (or being delivered locally). You can enable as much verbosity as you need in the logs, like the interfaces the mail passes through. Each line prefixes the message ID.
Postfix is also good here, but for Qmail you have to guess which line belongs to which email.

Exim does not need any config or list files to be compiled into db format. /etc/aliases is read as is, and it's fast too.

I doubt that Exim is the world standard. I only Exim due to DirectAdmin using it. I couldn't find any statistics about SMTP server usage. Please show some proof dat Exim is the most used SMTP server. I highly doubt it.

In my surrounding I see lots and lots of migrations to Postfix. It's easier to manage, has better tools.

Since we already use exim in other parts of our company, it would be easier to maintain. There are different criterias for each mail server, but both of them, postfix and exim, have advantages.
One thing though is that exim supports SNI which would make it possible to customers to have their certificate not only on the website but also on the mail server!

Exim is kind of World Standart right now. Even some antispam companies use EXIM for their MTA.
So because it is widely used there are lots of feedbacks and find solutions how to fix problems etc.
On the other hand I would never prefer to go with 2 MTA at the same time. İt would be the worst solution.

I dont see why Plesk should consider Exim. Postfix is a great MTA and I dont see what adding Exim to Plesk has to offer.

Besides that, it also means that any new future related to e-mail has to be implemented in 2 MTA's which means more development time. It is also possibel that a certain feature is not available in one of these MTA's, which means you would have to drop the feature completely.

IMHO bad idea as it means more work, and I dont see any benefits of adding another MTA.